The document defines and classifies different types of antigens. It discusses complete antigens that can induce antibody formation on their own and incomplete antigens or haptens that require a carrier protein to become immunogenic. It also outlines 10 key properties that determine antigenicity, including foreignness, size, chemical nature, susceptibility to enzymes, and specificity based on species, isoantigens, organs, and heterophile antigens. Certain self-antigens like lens proteins and sperm are not recognized as self due to limited exposure to the immune system during development.

1. By- annu maurya

2.  An antigen is a substance which , when introduced into
a body evokes immune response to produce a specific
antibody with which it reacts in an observable manner.

3.  They may be classified as
 A. Complete Antigen
 B. Haptens/Incomplete Antigen: Complex haptens;
Simple haptens.

4.  These are substances which can induce antibody
formation by themselves and can react specifically with
these antibodies.

5.  Haptens are substances unable to induce antibody
formation on its own but can become immunogenic
(capable of inducing antibodies) when covalently
linked to proteins, called carrier proteins.

7.  1. Foreignness
 2. Size
 3 . Chemical nature
 4. Susceptibility to tissue enzymes
 5. Antigenic specificity
 6. Species specificity
 7. Isospecificity

8.  8 . Auto specificity
 9. Organ specificity
 10 . Hetero phile specificity.

9.  An antigen must be foreign to the individual (non-self)
to induce an immune response. The immune system
does not normally mount an immune response against
his own antigens (self). This tolerance to self antigens
is due to continuous contact with them during the
developmental stages of immune system. However,
breakdown of this mechanism results in autoimmune
disease.

10.  Larger molecules are highly antigenic whereas low
molecular weight (less than 10,000) are either
nonantigenic or weakly antigenic. Substance with low
molecular weight may be rendered antigenic by
adsorbing these on inert particles like bentonite or
kaolin.

11.  Most naturally occurring antigens are either proteins or
polysaccharides. Proteins are more effective antigens.
An exception is gelatin which is non-antigenic protein
due to its low tyrosine (aromatic radical) content. The
presence of aromatic radical is suggested as essential
for antigenicity. Lipids and nucleic acids are less
antigenic on their own but do so when combined with
proteins.

12.  Substances which can be metabolised and are
susceptible to the action of tissue enzymes behave as
antigens. The antigen introduced into the body is
degraded by the enzymes of the phagocytic cells into
fragments of appropriate size containing antigenic
determinants. The hidden antigenic determinants can be
identified only when molecule is broken down . Bovine
serum albumin (BSA) has more than 18 determinants
although only six of these are exposed in the intact
(unbroken) molecule.

13.  Antigenic specificity depends on antigenic
determinants. The position of antigenic determinant
group in the antigen molecule is important for
specificity. The differences in specificity are
determined in compounds with the group attached at
ortho , meta or para positions. Another factor is spatial
configuration of the determinant group which makes
differences in antigenic specificity of dextro, laevo and
mesa isomers of substances.

14.  Tissues of all individuals in a species possess species
specific antigens. Some degree of cross reaction exists
between antigens from related species. The species
specificity has got phylogenetic relationship and has
been of assistance in (i) tracing of evolutionary
relationship (ii) forensic applications in the
identification of species of blood and of seminal fluid.

15.  Isospecificity depends on isoantigens which may be
found in some but not all members of a species . A
species may be grouped according to the presence of
different isoantigens in its members. These are
genetically determined. Blood grouping is one example
depending on human erythrocyte antigens
(isoantigens).

16.  Self antigens are generally nonantigenic, but there are
exceptions. These exceptions are lens protein and
sperm. These are not recognised as self antigens. Lens
protein normally confined within its capsule doesn't
come in contact with immune system, therefore, it is
not recognised. Similarly, antigens (sperm) that are
absent during the embryonic life and develop later, are
also not recognised by immune system.

17.  Organ specific antigens are confined to a particular
organ. Some organs like brain, kidney and lens protein
of one species share specificity with that of another
species. As brain specific antigens are shared by man
and sheep, antirabies vaccines (sheep brain vaccines)
may cause neuroparalytic complications in man.
Antirabies vaccine (developed in sheep brain) contains
large amount of partially denatured brain tissue.

18.  The same or closely related antigens present in
different tissues of more than one species are known as
heterophile antigens. Antibodies to these antigens
produced by one species cross react with antigens of
other species.

19.  Superantigens are molecules that can interact with
antigen-presenting cells (APCs) and T-lymphocytes in a
non-specific manner. These antigens do not involve the
endocytic processing as required in typical antigen
presentation.